1. Field of the Invention
The present invention relates to a high-density recording type tape-form magnetic recording medium. More particularly, the present invention is concerned with a tape-form magnetic recording medium which is advantageously used in a magnetic recording/reproducing system using a high sensitivity magnetic head (MR head or GMR head).
2. Description of Related Art
In recent years, in fields of video tape recorder and the like, for improving image quality and recording density, there has been proposed a so-called thin metal film type magnetic recording medium which comprises a magnetic layer formed by depositing a magnetic material, such as a metal material, a Co—Ni alloy, a Co—Cr alloy, or a Co—CoO metal oxide, directly on a nonmagnetic support using a vacuum thin film formation technique. As the magnetic recording medium, for example, deposited tapes for high band 8 mm video tape recorder and digital video tape recorder are practically used.
The thin metal film type magnetic recording medium is excellent in its coercive force and squareness ratio, and has excellent electromagnetic conversion properties since its magnetic layer can be made extremely thin in a short wavelength region, and is extremely low in recording demagnetization or thickness loss during reproduction. In addition, the thin metal film type magnetic recording medium has various advantages, for example, in that, differing from a coating type magnetic recording medium, it does not contain a binder which is a nonmagnetic material in the magnetic layer, and hence a packing density of ferromagnetic metal fine particles can be increased.
In accordance with an increasing demand for magnetic recording medium, such as a magnetic tape, as a data streamer, development of magnetic recording medium having even higher recording density is desired. Recently, as a magnetic head for reproducing recorded information, instead of a conventional inductive head, a magnetoresistive magnetic head (hereinafter, frequently referred to as “MR head”) or a giant magnetoresistive magnetic head (hereinafter, frequently referred to as “GMR head”) is used. The MR head can detect very slight leakage magnetic flux from the magnetic layer with high sensitivity, and therefore can improve the recording density.
The MR head or GMR head has a detection limit at which the sensitivity for leakage magnetic flux is saturated, and hence cannot detect the leakage magnetic flux higher than a range acceptable by the design of the head. Therefore, there is a need to reduce a thickness of the magnetic layer in the magnetic recording medium to optimize the leakage magnetic flux of the magnetic layer. Generally, in a magnetic recording tape system, if deterioration of the magnetization amount of a tape is 16% or more, a satisfactory reproducing signal cannot be obtained due to the too large deterioration. For this reason, the system is formed on an assumption that deterioration of the tape magnetization amount is 15% or less.
Generally, in a magnetic recording medium, such as a metal evaporate tape, for the purpose of improving the storage properties and running durability of the magnetic layer, a protective film comprised of a diamond-like carbon (DLC) is formed on the magnetic layer. The protective film comprised of the DLC has excellent density, as compared to a protective film comprised of carbon deposited by another vacuum thin film formation technique, such as a sputtering process.
Deterioration of the magnetic layer is advanced by oxidation of a magnetic metal due to oxygen or moisture present in an atmosphere. A protective film having high density inhibits oxygen or moisture in an atmosphere from diffusing in the magnetic layer. Therefore, in a case where the protective film comprised of the DLC is formed, the storage properties of the magnetic layer can be drastically improved.
However, it is known that the magnetic layer suffers corrosion due to moisture or air which has passed through the nonmagnetic support, so that the electromagnetic conversion properties and durability of the magnetic recording medium deteriorate (see Patent documents 1 and 2). For preventing the corrosion of the magnetic layer from a nonmagnetic support side, in the magnetic recording medium described in Patent document 1, a metal film having a metal oxide film on its surface is provided as an undercoat film between the ferromagnetic metal thin film comprised mainly of Co and the nonmagnetic support. In the undercoat film, a metal which is baser than Co or a metal oxide is used. In the metal film in the undercoat film, a metal, such as Al, Zn, Mn, V, Zr, Cr, In, or Tl, is used, and, in the metal oxide film, an oxide of the above metal is used.
Patent document 1 states that a thickness of the metal film is preferably 50 to 200 nm, further preferably 80 to 150 nm, and that a thickness of the metal oxide film is preferably 2 to 100 nm, further preferably 4 to 25 nm. In addition, there is a description showing that a thickness of the entire magnetic layer is preferably 120 to 300 nm and the output can be satisfactorily increased in this case.
In the magnetic recording medium described in Patent document 2, a base film comprised of at least one metal selected from Al, Zn, Mn, V, Zr, Cr, Fe, In and Tl is formed between the nonmagnetic support and the ferromagnetic thin metal film, and further, on the ferromagnetic thin metal film, a plasma polymerized film containing C and H is formed as a top coat film. Patent document 2 states that a thickness of the base film is preferably 50 to 150 nm, further preferably 80 to 120 nm, and that the ferromagnetic metal thin film is preferably comprised mainly of Co. Like the magnetic recording medium described in Patent document 1, there is a description showing that the thickness of the entire magnetic layer is preferably 120 to 300 nm and the output can be satisfactorily increased in this case.
On the other hand, as a method for preventing corrosion of the magnetic layer due to moisture or a corrosive gas, there has been known a method in which a layer comprised of aluminum (Al), chromium (Cr), titanium (Ti), or copper (Cu) having a thickness of 5 to 300 nm is formed under the magnetic layer (see, for example, Patent document 3). In Patent document 3, the thickness of the magnetic layer is as large as, for example, 200 nm, and the corrosion resistance to SO2 gas which is a corrosive gas is evaluated.
[Patent Document 1]
Japanese Patent Application Laid-Open Specification No. 4-335206 (Japanese Patent No. 3093818)
[Patent Document 2]
Japanese Patent Application Laid-Open Specification No. 5-20662
[Patent Document 3]
Japanese Patent Application Laid-Open Specification No. 1-124115
[Non-patent Document 1]
“Magnetoresistive head and spin valve head—Foundation and application—, 2nd edition”, translated by Kazuhiko Hayashi, 2002, published by Maruzen Co., Ltd.
As mentioned above, in the magnetic tape used in the system employing a high sensitivity magnetic head, such as an MR head, for preventing saturation of the magnetic head, the thickness of the magnetic layer is required to be small, for example, about 50 nm or less. In the GMR head having higher reproduction sensitivity than that of the MR head and being suitable for recording with high density at high frequencies, the thickness of the magnetic layer is required to be further smaller.
If the thickness of the magnetic layer is reduced, the magnetic layer is more likely to deteriorate. Therefore, even in a case where the base film described in Patent documents 1 and 2 is formed, deterioration of the magnetic layer cannot be satisfactorily prevented. In addition, in a case where the base film having the thickness described in Patent documents 1 and 2 is formed in a magnetic recording medium in which the thickness of the magnetic layer is as small as about 50 nm or less, the base film is too large in thickness and roughens a surface of the magnetic layer, thus increasing a medium noise as the magnetic recording medium. This problem is more serious when the GMR head having higher reproduction sensitivity than that of the MR head is used as a head for reproduction.
Patent document 3 has a description showing that, when a layer comprised of Al, Ti or the like is formed under the magnetic layer, the coercive force is improved, and, according to the below-described experimental results of the present inventors, it has been confirmed that, when a metal layer having a thickness as small as 30 nm or less is formed under the magnetic layer, the coercive force is lowered. However, when a metal layer having a thickness larger than that thickness is formed under the magnetic layer, an increase of the coercive force is also observed. In other words, when a range of the thickness of the magnetic layer or the base layer is changed, the change of the magnetic properties depending on the thickness of the base layer exhibits different behaviors.
Deterioration of storage properties at a high humidity at a high temperature is actualized when the thickness of the magnetic layer is 100 nm or less, and is different in a mechanism (reaction) of magnetization deterioration from corrosion caused by SO2 gas. In the actual experiments, there are often obtained results in which one of the storage properties at a high humidity at a high temperature and a corrosion resistance to SO2 gas is very excellent and another is poor. That is, the method for improving the corrosion resistance to SO2 gas is not always effective in improving the storage properties at a high humidity at a high temperature.
The present invention has been made for solving the above-mentioned various problems, and the present invention provides a magnetic recording medium which advantageously increases the recording density, and which effectively prevents deterioration of the magnetic layer due to the storage.